1. Field of the Invention
The present invention relates to color processing for calculating colorimetric data under a given viewing condition.
2. Description of the Related Art
As a technique of uniformly managing the color gamuts and color reproduction characteristics of a plurality of different devices and accurately transferring them, a color management system (to be abbreviated as CMS hereinafter) is known. In order to manage the color reproduction characteristics of respective devices, a CMS outputs the color reproduction characteristic data of the respective devices, for example, predetermined color patches in the case of a printer, and uses a device profile which describes the data obtained by colorimetrically measuring the color patches, thereby implementing color management. In order to generate a device profile, colorimetry using a spectral radiancemeter or spectrophotometer is necessary to obtain the color reproduction characteristic data of the respective devices.
The feature of colorimetry by a spectrophotometer and that of colorimetry by a spectral radiancemeter are described now.
FIG. 1 is a view schematically illustrating colorimetry by a spectrophotometer 15. In colorimetry by the spectrophotometer 15, a sample 11 (e.g., a color patch) is irradiated with light from a specific direction, and the light reflected in a specific direction is measured. In such colorimetry, since the geometric condition of an illuminant 13 and that of a photoreceptor 12 can be fixed, it is possible to place the illuminant 13 and photoreceptor 12 in a relatively small colorimetry unit 10 and cause the colorimetry unit 10 to scan, so as to colorimetrically measure many samples 11 in an automatic manner.
Colorimetry by the spectrophotometer 15 is contact type colorimetry, so it is advantageously simple. However, since light is measured under a specific geometric condition, the color appearance of the sample 11 under the actual viewing condition of an observer cannot be colorimetrically measured with high accuracy.
FIG. 2 is a view schematically illustrating colorimetry by a spectral radiancemeter 14. In colorimetry by the spectral radiancemeter 14, the position of the radiancemeter can be adjusted in accordance with the actual viewing condition (geometric condition) of the observer. Accordingly, while light from a plurality of illuminants 13 is scattering, it is possible to measure the light reflected from the sample 11 to the light-receiving direction of the observer. That is, it is possible to obtain a colorimetric value close to the color appearance of the sample 11 under the actual viewing condition of the observer.
The spectral radiancemeter 14 has the advantage of being capable of colorimetrically measuring the color appearance under a viewing condition with a high accuracy. However, since the spectral radiancemeter 14 is large compared to the spectrophotometer 15, it is not suitable for automatic colorimetry and requires a long colorimetry time. Even if a device capable of automatic colorimetry is available, since it requires a certain period of time to integrate colorimetric light, requires aligning the sample with the device in accordance the viewing condition (geometric condition), and the like, it still requires a long colorimetry time compared to the spectrophotometer 15.
As described above, in colorimetry by a spectrophotometer, the color appearance under an actual viewing condition cannot be colorimetrically measured with a high accuracy. On the other hand, colorimetry by a spectral radiancemeter requires a long colorimetry time. That is, it is difficult to colorimetrically measure the color appearance under an actual viewing condition with a high accuracy and easily in a short period of time.
Japanese Patent Laid-Open No. 2006-200960 discloses a means for solving this problem. A technique disclosed in Japanese Patent Laid-Open No. 2006-200960 corrects a colorimetric value under the first viewing condition to that under the second viewing condition. First, a white plate is colorimetrically measured under the first and second viewing conditions. Then, data for correcting a colorimetric value is generated from spectral distribution, the components of which vary depending on the viewing condition. By using this correction data, it is possible to calculate colorimetric data to be obtained upon colorimetry by a spectral radiancemeter in an environment close to the actual viewing condition, from the colorimetric data of a spectrophotometer. Accordingly, a time required for colorimetry decreases, and highly accurate colorimetric data which is close to that viewed under the actual viewing condition is obtained.
In the technique disclosed in Japanese Patent Laid-Open No. 2006-200960, however, it is necessary to generate correction data for converting the colorimetric data of the spectrophotometer into that of the spectral radiancemeter for respective actual viewing conditions. In other words, conversion of colorimetric data is possible only for the viewing condition for which correction data is generated. That is, when correction data for a new viewing condition is required, generation (colorimetry) of new correction data is necessary.
In addition, even if correction data for various viewing conditions are generated and a mechanism to hold them is prepared, it is impractical to cover all viewing conditions since correction data are different depending on the viewing conditions (the type of illuminant, geometric condition, media characteristics, color material characteristics, or the like).
Of course, an expensive spectral radiancemeter is necessary to generate correction data, and therefore it is impractical to generate correction data in a general user environment.